Purification of alloys



Oct. 19, 1965 E. J. DUNN, JR., ETAL 3,212,881

PURIFICATION OF ALLOYS Filed Dec. 4, 1962 R U F L U s OXYGEN CALCIUM ADDITION- WEIGHT "/0 INVENTORS Edward J. Dunn,Jr. 0nd Donoid M. Kelmun.

ATTO NEY WITNESSESI w @6- United States Patent 3,212,881 PURIFICATION 0F ALLQYS Edward J. Dunn, Jr., Snowden Township, Allegheny County, and Donald M. Kelman, Franklin Township, Westmoreland County, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 4, 1962, Ser. No. 242,214 5 Claims. (CI. 75-57) This invention is directed to the purification of ironbase and nickel-base alloys.

It has long been desired to reduce the level of sulfur and oxygen impurities in iron-base and nickel-base alloys. To accomplish the reduction of sulfur, various slags have been employed and additions have been made to minimize the detrimental effects of the sulfur content. The main sources of the sulfur found in the alloys are the raw materials and machining oils which accumulate on the recycled scrap. With conventional melting practices the degree of sulfur removal is limited. In normal induction melting practice, sulfur is not removed at all and the use of scrap is limited because of the possible contamination of the melt by sulfur.

Considerable effort has also been employed to remove oxygen from these alloys without contaminating the metal. In many common deoxidation practices the de oxidants used and their products of deoxidation are not completely removed from the melt and consequently may deleteriously affect the properties of the product.

Accordingly, it is an object of the invention to rapidly reduce the sulfur and oxygen contents of alloys to low levels during melting by using a purifying addition.

It is a further object of this invention to rapidly reduce the sulfur and oxygen contents of alloys to low levels during melting without seriously contaminating the alloys with other materials.

It is still a further object of this invention to rapidly reduce the sulfur content of iron-base and nickel-base alloys to low levels when the alloys are in a molten condition without seriously contaminating the alloy with other impurities.

Another object of the invention is to rapidly reduce the sulfur and oxygen contents of iron-base and nickelbase alloys to low levels when in the molten condition by the addition of a purifying element, and thereafter em loy a vacuum treatment to further purify the alloys.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a better understanding of the nature and objects of this invention,

reference should be had to the following detailed description and to the drawing, in which:

The figure is a graph in which sulfur and oxygen contents of an alloy are plotted against the amount of calcium added to the molten alloy.

Broadly, this invention is concerned with a process for employing elemental calcium as an addition during the melting of alloys to reduce the sulfur and oxygen contents of the treated alloys.

More specifically, the process of this invention involves the treatment of iron-base or nickel-base alloys as follows: The molten alloy is provided with a slag cover capable of absorbing calcium-sulfur compounds and preventing their reversion to the bath, from 0.5% to 2%, by weight, and preferably from 0.6% to 1%, by weight, of elemental calcium is added to the bath whereby the oxygen and sulfur in the bath are reduced to low levels, and thereafter the allow heat is poured.

The above process will produce alloy heats having only very small amounts of oxygen and sulfur remaining therein. However, the alloy heats so made will also probably contain small amounts of calcium, of the order of perhaps 0.03% by weight. In some applications alloys containing this residual calcium will be perfectly satisfactory, but if the alloy is to be forged even this small quantity of calcium is undesirable. It has been found that the calcium tends to form a low melting point constituent which is damaging to the forgeability of the alloys.

Accordingly, steps to reduce the residual calcium content are another feature of this invention. Reduction of the calcium in the molten alloy may be accomplished in three ways which may be employed separately or in combination: These methods for reducing calcium content are as follows:

(1) Flotation, which involves holding the alloy at temperature in a molten condition to permit the calcium to rise to the surface to combine with the slag;

(2) Oxidation, which involves exposing the molten alloy to air or other oxidizing atmosphere to oxidize the calcium whereupon it will rise to the surface of the alloy heat;

(3) Vacuum treatment, which involves placing the molten alloy in a vacuum environment in which the calcium is removed from the alloy by vaporization.

As a method for removing calcium from the alloy, flotation is quite slow and will not consistently reduce calcium values to the levels desired in reasonable times. Therefore, while flotation is useful in reducing calcium content, generally it will not be solely relied upon to produce the low calcium values, but will be used in combination with the other methods described.

Oxidation of the alloys by exposure to air is quite effective in reducing the calcium content. This may be done by removing the slag cover from the alloy and holding, or it may be done by pouring the alloy in air several times, i.e., furnace to ladle, ladle to furnace, furnace to ladle, and finally, ladle to ingot mold. This method does have the disadvantage that the alloy heat tends to pick up oxygen during its exposure to air, nullifying to some extent the deoxidat-ion effected earlier by the calcium.

Vacuum treatment of the calcium treated alloy heat has produced a most satisfactory high-purity product. The low-sulfur calcium treated alloy heat is poured and permitted to solidify in the form of an electrode. The electrode is then consumably arc melted in vacuum which reduces the calcium content to a few parts per million and further reduces the oxygen content.

In carrying out this invention the charge is first melted and after melting is raised to a superheated condition (of perhaps C. over the melting point) to prevent freezing, and a lime slag is added to the melt. This slag consists primarily of lime (CaO) and may also contain fluorspar (Cal-'3) to assure adequate fluidity. Relatively small heats of up to 1000 pounds have been made without using fluorspar. An addition of fluorspar in the amount of 10% of the slag weight proved satisfactory in making a 5000 pound heat. Sufiicient lime is employed to completely cover the alloy bath and to provide a volume sufficient to adequately absorb the calciumsulfur compounds formed. In one case, quantities equivalent to 1.75 to 2% of the charge weight have been found to be satisfactory.

The elemental calcium, which has been fastened securely to iron rods or rods of materials compatible with the melt of approximately /2" to diameter is then plunged deeply into the melt. The reactions which result may include the following:

Ca+ Q CaO Ca+- CaS More complex reactions may occur in addition to or in preference to the above. These reactions effectively and immediately reducethe sulfur and oxygen in the molten alloy to levels as low as .0008% sulfur and 3 p.p.m. of oxygen. More consistent results have been obtained at levels of .001% sulfur and p.p.m. of oxygen. The excess calcium may thereafter be removed by the methods which have been described above.

The following examples show the purification results obtained in the melting of an iron-nickel-cobalt alloy containing approximately 53% iron, 29% nickel and 17% cobalt.

EXAMPLE I A series of fifty pound heats of the 53% iron, 29% nickel and 17% cobalt alloy are melted in a magnesia crucible using virgin raw materials which included 0.02% by weight of sulfur. The alloy is superheated to a temperature about 100 C. above the melting point. A slag addition is then made which consists of lime (CaO) equal to 1.75% by weight of the charge which is sufficient to completely cover the molten metal. The addition of calcium is made by a submersion technique which insures maximum metal-to-addition contact. This technique consists of wrapping the calcium additions with aluminum screening or expanded metal, attaching the additions to commercially pure iron rods and plunging them deep into the meal bath. Varying amounts of calcium are added to the series of heats. Following the addition of calcium the slag is poured off and the heat allowed to stand for about 10 minutes to permit the residual calcium to react with the oxygen of the air to form CaO and separate and rise to the surface of the alloy. Thereafter, the heat was poured into an ingot mold and permitted to solidify. The sulfur and oxygen residuals in each of the fifty pound heats were determined by conventional techniques (oxygen-vacuum fusion, sulfur-combustion and iodate titration) and the data was then used to construct curves representing the sulfur and oxygen residuals versus the percent calcium added.

In the figure, curves 1 and 2 are the result of plotting the residual sulfur ad oxygen contents in the alloy of Example I against the calcium addition and show that a calcium addition or from about 0.6% to 1% is extremely effective in reducing the sulfur and oxyen contents.

There follows a table in which the results obtained in Example I are summarized.

Table I.Efiect of calcium additions on 53% Fe, 29% Ni, 17% Co, alloy 1 Poor sample (blown-analysis not reliable). 2 Part of the Ca additions were not added effectively.

An effort was made to forge Heat No. DX-380, but the alloy broke up severely in the forging process. This heat was then remelted under a lime slag cover, the slag was removed, and the heat held for 10 minutes at a temperature of 1600 C. After solidification to ingot form, the alloy was successfully forged. The success in forging the remelted heat is attributed to the reduced amounts of residual calcium.

EXAMPLE II Following the practice outlined in Example I, a number of 5500 pound melts of the 53% iron, 29% nickel and 17% cobalt alloy were melted and purified with calcium additions. In these heats about 0.76% calcium addition was employed. The results obtained on these larger heats fully reproduced the results predicted from the experience with the fifty pound heats. The amounts of sulfur and oxygen remaining in the heats after treatment was drastically reduced to levels similar to those obtained in the heats of Example I employing similar quantities of calcium.

Some of these larger heats were analyzed for residual calcium. Calcium was shown to be present in amounts of the order of over 200 p.p.m. in heats to which 0.76% calcium had been added when the heats were poured without holding at temperature. Residual calcium in amounts in excess of 200 p.p.m. has been found to be particularly deleterious. However, when holding times were employed residual calcium levels of about 30 to 60 p.p.m. and improved working characteristics resulted.

Since removal of calcium is necessary to assure forgeability, and since it is also desirable to keep oxygen content at low levels, a vacuum treatment was determined upon to reduce the quantity of both of these elements in treated alloys. The following example sets forth the results achieved.

EXAMPLE III A 5500 pound alloy heat of an iron-nickel-cobalt alloy is melted according to the practice of Example I. Following the calcium addition and its reaction with the sulfur and oxygen in the alloy, the alloy contains approximately 250 p.p.m. of calcium and about 10 p.p.m. of oxygen. The heat is held at temperatures and under its slag cover for a period of about 45 minutes and then is poured into a mold to form an electrode. The analysis of the electrode is shown below in Table II. The electrode is then consum-ably arc melted in vacuum and the resultant ingot is again analyzed. The analysis of the vacuum arc melted ingot is also shown in Table II. The values in Table II are in Weight percent except as otherwise indicated.

Table II Vacuum Electrode Melted Ingot (Wt. Percent) 45 Bal. Bal.

*Average of two analysis.

The vacuum treated ingot was forged and displayed excellent forgeability.

It will be noted from Table II that the calcium content decreases while the oxygen content rises during the holding period. The vacuum treatment drastically reduces the amount of calcium remaining in the alloy.

In addition to the iron-nickel-cobalt alloy which has been described, the invention has been successfully practiced on a 3% silicon-iron alloy and an nickel, 4% molybdenum, 0.35% silicon, balance iron, alloy. Other iron-base or nickel-base alloys may be similarly treated.

The slag practices employed in basic arc furnace steel making, under highly controlled conditions, can be used to reduce sulfur to values of the order of 0.005% However, this method is time consuming and it is quite difficult to obtain reproducibility. Following the method of calcium addition in accordance with the present invention, sulfur residuals in the order of 0.002% to less than 0.001% can be obtained very rapidly.

Calcium can also reduce oxygen to residuals equal to or lower than that produced by vacuum melting. For example, oxygen residuals of 4 to 30 p.p.m. are reported for vacuum melting while following the calcium addition method of this invention oxygen residuals of 10 p.p.m.

can be readily and consistently obtained without vacuum processing. Such heats are useful if forgeability is not an important consideration.

While only one specific method of vacuum treatment is set forth above, it will be understood that the molten alloy may be exposed to a high vacuum in a variety of ways with satisfactory results. Thus, the molten alloy which may be air melted, may be teemed into a vacuum chamber. The apparatus shown in US. Patent No. 2,885,751, issued May 12, 1959, is one suitable apparatus for accomplishing this. Another method of exposure to vacuum is simply to remelt the calcium treated alloy in a vacuum induction furnace. Still another method is to remove the slag cover from the air melted molten alloy and place the molten alloy in a chamber which is then evacuated. All of these methods for exposing the molten alloy toa vacuum environment are effective in removing calcium from the alloy. A vacuum of at least the order of from 1 to microns, and preferably 1 micron or less is suitable.

The inventive principles embodying the above description and the figure may obviously be incorporated in modified processes by those skilled in the art without departing from the spirit and scope of this invention, and it is intended that the description be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In a process for producing low oxygen and low sulfur contents in air melted alloys selected from the group consisting of iron-base and nickel-base alloys, the steps comprising, providing the molten alloy with a lime slag cover capable of absorbing calcium and calcium-sulfur compounds and preventing their reversion to the bath, adding from 0.5% to 2%, by weight, of elemental calcium .to the bath whereby the oxygen and sulfur in the bath are reduced to low levels, holding the molten alloy heat under its slag cover for a time sufficient to permit the reduction of the calcium content of the molten alloy by flotation wherein the calcium rises to join the slag cover, pouring the heat and permitting it to solidify, remelting the alloy heat in a vacuum environment to further reduce the calcium content of the molten metal and furthe reduce the oxygen content thereof, and thereafter permitting the alloy heat to solidify.

2. The process of claim 1 wherein the calcium treated alloy heat is first solidified in the form of an elect-rode, and said electrode is consumably arc melted in vacuum.

3. The process of claim 2 wherein the calcium addition is made in the amount of from 0.6% to 1% by weight.

4-. In a process for producing alloys having extremely low sulfur and oxygen contents, the steps comprising, melting the alloy constituents, providing the molten alloy heat with a lime slag cover capable of absorbing calciumsulfur compounds and preventing their reversion to the bath, adding from 0.5% to 2%, by weight, of elemental calcium to the molten alloy to reduce the oxygen and sulfur in the alloy to low levels, holding the molten alloy under the slag cover for a time sufficient to permit reduction of the calcium content of the molten alloy by flotation wherein the elemental calcium rises to the surface of the alloy and join the slag cover, and thereafter exposing the molten alloy to a high vacuum by first solidifying the calcium treated alloy and then remelting the solidified alloy in a vacuum induction furnace whereby the calcium content of the alloy is further reduced and additional oxygen is also removed from the alloy, and thereafter pouring the molten alloy.

5. The process of claim 1 in which the heat is poured at least twice in air prior to the initial solidification to obtain oxidation of the calcium.

References Cited by the Examiner UNITED STATES PATENTS 1,277,523 9/18 Yensen 49 1,658,879 2/28 Browne 7558 1,922,037 8/ 33 Hardy 7558 2,096,317 10/37 Browne 75-129 2,113,021 4/38 Greenidge 7557 2,144,200 1/39 Rohn et al. 7557 2,804,387 8/57 Margan et a1. 75-129 2,837,790 6/58 Rozian 7549 2,899,294 8/ 59 Siemons 7549 2,930,690 3/60 Meinen 75129 FOREIGN PATENTS 759,292 10/56 Great Britain.

BENJAMIN HENKIN, Primary Examiner. 

1. IN A PROCESS FOR PRODUCING LOW OXYGEN AND LOW SULFUR CONTENTS IN AIR MELTED ALLOYS SELECTED FROM THE GROUP CONSISTING OF IRON-BASE AND NICKEL-BASE ALLOYS, THE STEPS COMPRISING, PROVIDING THE MOLTEN ALLOY WITH A LIME SLAG COVER CAPABLE OF ABSORBING CALCIUM AND CALCIUM-SULFUR COMPOUNDS AND PREVENTING THEIR REVERSION TO THE BATH, ADDING FROM 0.5% TO 2%, BY WEIGHT OF ELEMENTAL CALCIUM TO THE BATH WHEREBY THE OXYGEN AND SULFUR IN THE BATH ARE REDUCED TO LOW LEVELS, HOLDING THE MOLTEN ALLOY HEAT UNDER ITS SLAG COVER FOR A TIME SUFFICIENT TO PERMIT THE REDUCTION OF THE CALCIUM CONTENT OF THE MOLTEN ALLOY BY FLOTATION WHEREIN THE CALCIUM RISES TO JOIN THE SLAG COVER, POURING THE HEAT AND PERMITTING IT TO SOLIDIFY, REMELTING THE ALLOY HEAT IN A VACUUM ENVIRONMENT TO FURTHER REDUCE THE CALCIUM CONTENT OF THE MOLTEN METAL AND FURTHER REDUCE THE OXYGEN CONTENT THEREOF, AND THEREAFTER PERMITTING THE ALLOY HEAT TO SOLIDIFY. 